1. Field of the Invention
Nuclear power plants, because of the potential accidental release of radioactive materials, are required by practice to be designed in such a manner that the health and safety of the public is assured even in the event of the most adverse accident that can be postulated. In nuclear power plants utilizing light water as a coolant, the most adverse accident possible is considered to be a double-ended break of the largest pipe in the reactor coolant system and such an accident is commonly termed the Loss Of Coolant Accident, hereinafter sometimes referred to as LOCA.
For accident protection, plants utilizing light water as the coolant employ containment systems designed to physically contain water, steam and any entrained fission products that may escape from the reactor coolant system. The containment system is normally considered to encompass all structures, systems and devices that provide ultimate reliability and complete protection for any accident that may occur. Engineering safety systems are specifically designed to mitigate the consequences of an accident, and the design goal of a containment system is that no radioactive material will escape from the nuclear power plant in the event of an accident.
The passive containment system disclosed herein provides this desired level of protection for a loss of coolant accident and for other types of accident that are considered as a basis of design, and the concepts of the invention are considered to be effective for nuclear power plants employing either pressurized water reactors or boiling water reactors.
2. The Prior Art
In order to provide containment for light water cooled nuclear power plants prior art techniques have basically utilized either full-pressure "dry-type" containment or pressure suppression containment.
In a full-pressure containment the reactor building, completely enclosing the reactor coolant system, is capable of withstanding the pressure and temperature rise expected to occur in the event of a LOCA. The builidng is usually constructed either of steel or steel-lined reinforced concrete or prestressed concrete.
Full-pressure containment systems may include double leakage control barriers and subatmospheric pressure operation. For the double leakage-control barrier any leakage into the control annulus is either pumped back into the primary containment, or the leakage is treated before being exhausted to the outside atmosphere. For subatmospheric operation the containment is normally maintained at partial vacuum, and following the LOCA, the pressure is reduced back to less than the outside atmosphere utilizing active engineered safety systems to terminate any potential release of radioactivity to the environment.
The pressure-suppression containment consists of a drywell that houses the reactor coolant system, a pressure-suppression chamber containing a pool of water, and a vent system connecting the drywell to the pool of water. This containment structure is constructed of steel enclosed by reinforced concrete, or is steel-lined with reinforced concrete. The pressure-suppression containment is housed within a reactor building. In the event of a LOCA, the reactor coolant partially flashes to steam within the drywell, and the air, steam and liquid coolant flow through the connecting vents into the pool of water in the suppression chamber. The steam is condensed by the water and decreases the potential pressure rise in the containment. The air rises into the free space above the pool of water in the suppression chamber.
Refinements in pressure-suppression containment utilizing water includes the inerting of the containment atmosphere. Inerting is aimed at preventing the burning of hydrogen produced from metal-water reaction of overheated nuclear fuel.
A different type of pressure-suppression containment utilizes an ice-condenser. The ice is maintained in a refrigerated compartment surrounding the reactor coolant system. The ice-condenser containment is divided into an upper chamber and a lower chamber with the reactor coolant system in the latter. In the event of a LOCA a pressure rise of the lower chamber causes access panels located at the bottom of the ice-storage compartment to open. This provides a flow path for air and steam through the ice bed. The steam is condensed by the ice and decreases the potential pressure rise in the containment. The air passes into the upper chamber through top access panels forced open by the flow of air.
Full-pressure containment and pressure-suppression containment are passive structures that require support systems for containment of the accident. Active systems such as residual heat removal systems and containment spray systems are used to dissipate heat to the environs. This prevents the containment design pressure and temperature from being exceeded and in the process, the containment pressure is reduced to limit the leakage of fission products. Active filtration systems are required in conjunction with the spray systems to reduce fission product concentration in the containment atmosphere. This also limits the amount of fission products that can leak out of the containment to the environs. Hydrogen recombiners are also being utilized to protect the containment from developing explosive concentrations of hydrogen.
To be effective, both the full-pressure containment and the pressure-suppression require additional engineered safety systems that provide emergency cooling of the nuclear fuel. Pressurized water reactors require passive accummulator systems in addition to active high and low pressure injection systems to maintain an adequate amount of liquid coolant at the nuclear fuel. The residual heat removal systems used for containment pressure reduction also reject decay heat to the environs.
Pressure suppression with gravity flooding has also been proposed as an engineering safety system for a LOCA.
Active engineered safety systems are inherently required to function effectively in order to maintain the integrity of the containment system in the LOCA. Active systems require high integrity instrumentation and control equipment, rotating machinery, electric power sources and power distribution equipment. These systems need to function properly as part of a larger system under adverse containment environment conditions of high-pressure, high-temperature, high-humidity, high-radioactivity, and eroded thermal insulation.
Malfunctioning of any active engineered safety system imposes even more adverse conditions on the operable system. For instance, an inadequate sourch of electric power may result in the malfunctioning of the emergency core cooling system for the nuclear fuel. Overheating of the fuel can result in melting of the fuel cladding with metal-water reactions occuring. The fuel core may slump and portions could collapse and overheat the bottom of the reactor vessel. Hydrogen is released from metal-water reactions and is subject to burning. The added energy from the metal-water reactions and from the burning of hydrogen imposes even more severe requirements on containment structure. Overheating of the fuel and melting of the cladding results in a gross release of fission products that are available for leakage from the containment system. This example points to the critical nature of active engineering safety systems that are an essential part of the containment system of the prior art.
The prior art has proposed a variety of solutions to the containment of a nuclear power plant in the event of a LOCA, and in my U.S. Pat. Nos. 3,984,282 and 4,050,983, I have proposed passive containment systems for confinement of the coolant in the event of a LOCA, and for cooling the reactor assembly in the event of such an accident. Further, in my U.S. Pat. No. 3,865,688 I have disclosed a passive confinement system utilizing many of the concepts herein set forth, and this invention constitutes an improvement over that specifically set forth in U.S. Pat. No. 3,865,688.